46
A BSR serves as the administrative core of the PIM-SM domain. A PIM-SM domain can have only one BSR,
but can have multiple C-BSRs. If the BSR fails, a new BSR is automatically elected from the C-BSRs to
avoid service interruption. A device can serve as a C-RP and a C-BSR at the same time.
As shown in
, each C-RP periodically unicasts its advertisement messages (C-RP-Adv messages)
to the BSR. A C-RP-Adv message contains the address of the advertising C-RP and the multicast group
range that it serves.
The BSR collects these advertisement messages. It then chooses the appropriate C-RP information for
each multicast group to form an RP-set. The RP-set is a database of mappings between multicast groups
and RPs. The BSR then encapsulates the RP-set in the BSMs that it periodically originates and floods the
bootstrap messages to the entire PIM-SM domain.
Figure 21
BSR and C-RPs
Based on the information in the RP-sets, all routers in the network can calculate the location of the
corresponding RPs based on the following rules:
1.
The C-RP with the highest priority wins.
2.
If all the C-RPs have the same priority, their hash values are calculated through the hashing
algorithm. The C-RP with the largest hash value wins.
3.
If all the C-RPs have the same priority and hash value, the C-RP that has the highest IP address wins.
The hashing algorithm used for RP calculation is "Value (G, M, C
i
) = (1103515245 * ( (1103515245 * (G
& M) + 12345) XOR C
i
) + 12345) mod 2
31
."
Table 7
Values in the hashing algorithm
Value Description
Value
Hash value.
G
IP address of the multicast group.
M
Hash mask length.
C
i
IP address of the C-RP.
&
Logical operator of "and."
XOR
Logical operator of "exclusive-or."
Mod
Modulo operator, which gives the remainder of an integer division.